Jack- The Decco is a very versatile unit and as you know can be used as a preamp, which might just be the real strength of this product. Digital amps can be tricky. Depending on your taste and based on the statement about the lack of sweet midrange sound, you may want to lean toward tubes for power. For an amp in your price range ($1350 new and even cheaper on the Gon), I think the Rogue Atlas would be a perfect mate to the D5/Decco combination.
One thing you have with the Era's is they are not the most efficient or speediest speakers made and the Decco's digital power may be a bit less refined and a tad lean to make them push as much air as they are capable of. You could consider a higher dB speaker, however I do like the sound of both the D4 and the D5s and have personally auditioned both of them with the Atlas and let me tell you they match up very, very nicely.
During my audition, various people walking into the store came into my room and 2 of them bought Era speakers based on this exact set up. The Eras are good, but paired with the Atlas they are excellent!
Although the Atlas is rated at 55 WPC, this is tube power which is much more deceptive than digital or SS power. As high quality, well priced amps go, the Rogue Atlas is probably as good as you will get at or near this price and I do like that it is made in the USA. The other thing to consider is popping in a quad set of JJ KT77s which will pump up the watts to about 70 WPC, although I think a new Atlas may now come stock with those tubes.
Paired with the Decco as a preamp which gives you the built in DAC, I think you will find this to be a very nice match up. Ultimately the addition of the ERA Sub 8 will round out the package very nicely.
The Rogue Atlas at its price range is a huge bargin and now that the dollar is falling like a rock into the deepest depths of the ocean, buying Made in USA can be a very good thing!
http://www.soundstage.com/revequip/rogueaudio_atlas.htm